Question

I have constructed in R a two variable integer valued function f(x,y) which is only well – defined for single entries (not for example f(1,1:5)). I am effectively looking for an integer valued function F(x,y,z,w) which would give the output:

f(x,y),f(x,y+1),...,f(x,w),f(x+1,y),f(x+1,y+1),...,f(x+1,w),...,f(z,y),f(z,y+1),...,f(z,w)

as a (z-x+1) by (w-y+1) matrix. Cheers for any help!

Answer 1

outer seems to be what you’re looking for here.

# make a simple function
f <- function(x, y){x+y}
x <- 3
z <- 5
y <- 2
w <- 7
outer(x:z, y:w, f)
#     [,1] [,2] [,3] [,4] [,5] [,6]
#[1,]    5    6    7    8    9   10
#[2,]    6    7    8    9   10   11
#[3,]    7    8    9   10   11   12

If it’s true that your function really only can take scalars as the input then you may need to use Vectorize to make this approach work

# Function that can only takes scalars...
f <- function(x, y){if(length(x) > 1 | length(y) > 1) stop('blah'); x + y}
outer(x:z, y:w, f)
#Error in FUN(X, Y, ...) : blah
myvectorizedfun <- Vectorize(f)
outer(x:z, y:w, myvectorizedfun)
#     [,1] [,2] [,3] [,4] [,5] [,6]
#[1,]    5    6    7    8    9   10
#[2,]    6    7    8    9   10   11
#[3,]    7    8    9   10   11   12